Background and Aims

Accurate dosimetry is paramount to study the FLASH effect since dose and dose-rate are critical dosimetric parameters governing its underlying mechanisms. With the goal of assessing the suitability of standard clinical dosimeters in a very high dose rate (VHDR) experimental setup, we evaluated the ion collection efficiency of several commercially available air-vented ionization chambers (IC) in conventional and VHDR proton irradiation conditions.

Methods

The Proteus 235 cyclotron (IBA) at the Orsay Proton therapy Centre was used to deliver VHDR pencil beam scanning irradiation. Ion recombination correction factors (k_s) were determined for several detectors (Advanced Markus, PPC05, RAZOR Nano, CC01) using Jaffé plots. Dose rate independent detectors such as a Faraday cup and alanine dosimetry were employed to cross calibrate absolute dose measurements of the ICs.

Results

Mean dose rates at isocenter ranged from 2Gy/s to 230Gy/s, and instantaneous dose rates were up to 1000Gy/s. The IC and Faraday cup agreed within 3% and differences between alanine and reference measurements were smaller than 1% for doses above 20Gy. Recombination correction factors below 1.5% were obtained for all chambers at VHDR with significant variations among detectors, while k_s values were significantly smaller (0.3%) for the conventional dose rates.

Conclusions

While the collection efficiency of the ICs in VHDR proton therapy is comparable to that in the conventional regime or for dose rates smaller than 150Gy/s, the reduction in collection efficiency cannot be ignored and varies significantly between irradiation conditions and detectors.

This work resulted from the project 18HLT04 UHDpulse which received funding from the EMPIR programme.